The present invention relates to a control blade for use in nuclear reactors for adjusting and controlling the power of a boiling water reactor or the like, and, more particularly, to an excellent reactivity and long-lived type control blade for nuclear reactors capable of controlling undesirable swelling of a neutron absorber or capable of preventing deterioration in the mechanical and physical life if swelling takes place.
Since the neutron absorption power (capacity) of a control blade for use in boiling water nuclear reactors is gradually deteriorated when it absorbs neutrons, the control blade must be taken out from the core of the nuclear reactor after it has been used for a predetermined period so as to be replaced by a new control blade. However, the above-described replacement work must be performed while shutting down the operation of the nuclear reactor, causing a large scale work to be performed which takes a too long time. Therefore, the time in which the operation of the nuclear reactor must be shut down takes a too long time, causing the availability to be deteriorated. What is even worse, there is a risk for operators to be exposed to radiation.
Furthermore, since the used control blade is a large and strong radioactive waste disposal, there recently arises a desire to lengthen the life of the control blade. As a result, a variety of novel long-lived type control blades for nuclear reactors have been developed, resulting a kind of them to be put into practical use.
The inventor of the present invention has promptly recognized the necessity of realizing a long-lived type control blade and thereby has disclosed a control blade for nuclear reactors for the purpose of lengthening the life thereof in Japanese Patent Laid-Open No. 53-74697 (Japanese Patent Publication No. 59-138987). The control blade for nuclear reactors according to the above-made disclosure is basically arranged in such a manner that a long-lived type neutron absorber exemplified by Hf metal or an Ag-In-Cd alloy is, in place of a boron compound, disposed in a portion which is exposed to a relatively large amount of neutrons.
A control blade for nuclear reactors is detachably inserted from a lower portion of the core into a gap having a cross-shaped lateral cross section and formed between fuel assemblies each of which is a set composed of four fuel assemblies loaded in the core portion of a nuclear reactor. The front portion of insertion and the outer end portion (the outer side end portion of the wing) of the control blade for nuclear reactors are exposed to a particularly large amount of neutrons. Therefore, hafnium (Hf), the neutron absorption power (capacity) of which cannot be largely deteriorated even if it is exposed to a large amount of neutrons, is disposed in the above-described portions so as to lengthen the life of the control blade for nuclear reactors. Furthermore, cheap and light boron carbide (B.sub.4 C) is disposed in the other portion of the control blade.
A conventional control blade for nuclear reactors is basically arranged as shown in FIGS. 34 to 36 in such a manner that a plurality of tie members 1 each of which has a cross-shaped connecting portion are disposed in the axial direction at predetermined intervals. Furthermore, four wings 2 each of which is formed into an elongated rectangular plate are secured to the above-described connecting member 1 in such a manner that the four wings 2 form a cross-shape. Reference numeral 3 represents a handle connected to the front portions of the four wings 2.
Each of the wings 2 has-a plurality of accommodating holes 2a formed in its widthwise direction, each of the accommodating holes 2a being formed in line in the lengthwise direction of the wing 2. In the accommodating hole 2a formed in the front insertion portion of the wing 2, a long-lived type neutron absorber 3a composed of Hf is disposed. Furthermore, a neutron absorber 4 made of B.sub.4 C powder is enclosed in the residual accommodating holes 2a. In the outer side end portion (periphery) of the wing 2 which corresponds to the outer periphery of the control blade A for nuclear reactors, a space 2b is formed in the lengthwise direction of the wing 2 in such a manner that it is communicated with each of the accommodating holes 2a. In the above-described space 2b, a long-lived type neutron absorber 3b composed of Hf is disposed to close an end portion of each of the accommodating holes 2a. The neutron absorber 3b acts to prevent the B.sub.4 C powder drop from the accommodating holes 2a. A stainless steel member 5 for use at the time of welding work is disposed between the neutron absorber 3b and the outer periphery of the wing 2.
When the wing 2 thus-arranged is assembled, the neutron absorbers 3a and 4 are inserted (injected) into the accommodating hole 2a through an opening portion 2c (FIG. 35) formed at a position confronting the outer periphery of the wing 2. Then, the neutron absorber 3b and the stainless steel member 5 are placed in the space 2b before a pair of plate portions 2d confronting each other in the opening portion 2c are bent inward so as to be closed by welding.
Since the melting point of the neutron absorber 3b made of Hf is 2200.degree. C. which is considerably higher than the melting point 1400.degree. C. of the stainless steel member 5, the neutron absorber 3b is not melted at the time of the welding work but is held in the outer periphery of the wing 2 made of stainless steel. Therefore, the undesirable mixture of Hf atoms of the neutron absorber 3b with the metal present in the portion to be welded can be prevented. As a result, sound weld portion can be obtained.
It is preferable that range 1.sub.1 (see FIG. 34) in which the long-lived type neutron absorbers 3a are disposed 3 cm or longer and as well as 35 cm or shorter from the front insertion portion of the wing which constitutes the control blade for nuclear reactor. If it is shorter than 3 cm, the B.sub.4 C powder which constitutes the neutron absorber 4 is undesirably placed in the region which is exposed to a large amount of neutrons. On the contrary, if it is longer than 35 cm, the reactivity worth will be deteriorated at the time of shutting down the nuclear reactor because Hf displays insufficient neutron absorption effect in comparison to B.sub.4 C. Furthermore, the expensive Hf is excessively used and its weight is heavy, causing an economical disadvantage to be taken place and the overall weight of the control blade for nuclear reactors to be increased undesirably in comparison to the conventional control blade for nuclear reactors.
It is preferable that the length of range 1.sub.2 of the neutron absorber 3b disposed in the outer periphery of the wing 2 be about 0.5 to, 2 cm. If it is shorter than 0.5 cm, the B.sub.4 C powder which constitutes the neutron absorber 4 is undesirably placed in the region which is exposed to a large amount of neutrons. If it exceeds 2 cm, Hf which constitute the neutron absorber 3b is undesirably placed in the region (the region adjacent to the central portion of the wing) in which the value of the neutron flux has been reduced, causing the problem similar to the above-made description to arise.
The inventor of the present invention has, in Japanese Patent Laid-Open No. 1-202691, disclosed a novel control blade for nuclear reactors arranged in such a manner its structure is similar to that shown in FIG. 34 and an Hf diluted alloy which is an alloy of Hf and zirconium (Zr) or an alloy of Hf and titanium (Ti) is used in place of stainless steel.
Hafnium forms an all-solid solution type alloy in association with Zr or Ti at an arbitrary ratio. Although a fact is well known that Hf, Zr and Ti are able to maintain satisfactory soundness in nuclear reactors, another fact is known that they absorbs hydrogen, thereby forms a hydride and will generate swelling, which is the cubical expansion, in a state where the quantity of hydrogen is excessively large but oxygen is insufficient. Hf is an excellent neutron absorber possessing multiple types of isotopes and it is known as a typical long-lived type neutron absorber because it is able to prevent the deterioration in the neutron absorbing performance even if it is exposed to neutrons. Furthermore, Hf has a characteristic of particularly absorbing resonance neutrons and its neutron absorption power (capacity) is not rapidly deteriorated even if it is diluted by a diluent to a certain degree.
As described above, although Hf possesses the excellent nuclear neutron-absorbing characteristic, it suffers from an excessively large density (about 13.1 g/cm.sup.3). Therefore, there arises a problem in that it cannot be easily employed as a material for the control blade for the conventional nuclear reactors.
On the other hand, Zr has a small density (6.5 g/cm.sup.3) and it is able to form an excellent alloy in association with Hf as described above. Also Ti has a small density (4.5 g/cm.sup.3) and it is able to form an excellent alloy with Hf. Therefore, an alloy of Hf and Zr or an alloy of Hf and Ti (since Zr or Ti serves as a diluent in view from Hf, they are called diluted alloy in this specification) is able to maintain excellent characteristics in nuclear reactors. Furthermore, the neutron absorption power is not deteriorated in comparison to the structure in which Hf is employed. Furthermore, the density of the diluted alloy can be reduced so that the density of the level possessed by stainless steel (about 8 g/cm.sup.3) can easily be realized. Therefore, a control blade for nuclear reactors which can be adapted to conventional nuclear reactors and which is constituted similarly to that shown in FIG. 34 can be constituted.
In this case, the stainless steel member for welding and represented by reference numeral 5 can be omitted from the structure shown in FIGS. 34 to 36. Therefore, it might be feasible to employ a member made of the above-described diluted alloy as the member for performing the welding work.
However, as described above, Hf or the Hf alloy will absorb hydrogen and generate swelling when an excessively large quantity of hydrogen is present and the quantity of oxygen is insufficient. The above-described novel control blade for nuclear reactors (see Japanese Patent Laid-Open No. 1-202691) is arranged in such a manner that accommodating holes are formed in the Hf diluted alloy sheet and the B.sub.4 C (boron carbide) powder is enclosed in the accommodating holes. .sup.10 B of B.sub.4 C reacts with neutrons to generate .sup.4 He and .sup.7 Li and as well as .sup.3 T (tritium).
If the amount of neutrons exposed to the control blade increases, the amount of .sup.3 T produced cannot be neglected. Although the major portion of .sup.3 T is left in B.sub.4 C, a portion of it is removed from B.sub.4 C. Since tritium is hydrogen, it can be absorbed by Hf, Zr or Ti. Therefore, there arises a problem in that the soundness of Hf, the Hf-Zr alloy or the Hf-Ti alloy which also serves as the structure member of the Control blade will be deteriorated.
The control blade for nuclear reactors of the type described above is arranged in such a manner that a multiplicity of the accommodating holes are formed in a sheet made of a diluted alloy prepared by diluting Hf by Zr or Ti serving as a diluent and the boron compound is enclosed in a portion or the major portion of the accommodating holes. There is a fear that tritium (.sup.3 T) produced when boron reacts with neutrons and hydrogen produced due to a radiolysis of water introduced at the manufacturing process move between accommodating holes and thereby hydrides HfH.sub.2 (Hf.sup.3 T.sub.2) , ZrH.sub.2 (Zr.sup.3 T.sub.2) ,TiH.sub.2 (Ti.sup.3 T.sub.2) or the like is produced with Hf, Zr or Ti. It leads to cubical expansion (swelling) due to the increase in the volume from the original Hf alloy. Therefore, there is a possibility that the stress will be generated in the Hf-Zr alloy or the Hf-Ti alloy from the inner surface of the accommodating hole, the Hf-Zr alloy or Hf-Ti alloy serving as the structure material.
Another long-lived type control blade has been disclosed by the inventor of the present invention in Japanese Patent Publication No. 1-45598 (Japanese Patent Laid-Open No. 57-171291). The control blade for nuclear reactors according to the above-made disclosure employs a neutron absorbing rod. The neutron absorbing rod is arranged in such a manner that boron carbide (B.sub.4 C) powder and a Hf metal rod or an Ag-In-Cd alloy rod are enclosed in an elongated covered pipe made of stainless steel. Furthermore, metal wool is disposed between the above-described two elements. In the front end portion which is exposed to a large amount of neutrons when viewed in the direction into which the control blade is inserted, the Hf metal rod or the Ag-In-Cd alloy rod is placed, while the B.sub.4 C powder is enclosed in the end portion opposing the front insertion portion because the amount of the neutron exposure is relatively small in this portion.
However, a fact has been found from the ensuing research that there is a room for improvement in the neutron absorbing rod, which is arranged in such a manner that both the boron compound and the Hf metal or the Ag-In-Cd alloy rod are enclosed. That is, a portion of tritium (.sup.3 T) produced as a result of boron-neutron reaction in the boron compound is absorbed into the surface layer of the Hf metal or the Ag-In-Cd alloy, which is sealed together, causing swelling to be generated in the neutron absorber. Therefore, the soundness of the covered pipe can be deteriorated.
Furthermore, a portion of water left in the covered tube at the time of the manufacturing process is changed into hydrogen due to radiolysis. Furthermore, since hydrogen is able to transmit the stainless pipe, hydrogen generated due to the radiolysis of the reactor core water can be introduced into the covered pipe. The above-described introduction of hydrogen will cause swelling since it can be absorbed into the surface layer of the Hf metal or the Ag-In-Cd alloy similarly to the above-described case about tritium.